1. Field of the Invention
The present invention relates to a method for producing a Group III nitride compound semiconductor. More particularly, the invention relates to a method for producing a Group III nitride compound semiconductor through lateral epitaxial overgrowth (LEO). As used herein, the term “Group III nitride compound semiconductor” refers to a semiconductor represented by the following formula: AlxGayIn1-x-yN (0≦x≦1, 0≦y≦1, 0≦x+y≦1), and encompasses two-component semiconductors such as AlN, GaN, and InN; three-component semiconductors such as AlxGa1-xN, AlxIn1-xN, and GaxIn1-xN (in each case, 0<x<1); and four-component semiconductors represented by the following formula: AlxGayIn1-x-yN (0<x<1, 0<y<1, 0<x+y<1). Unless otherwise specified, in the present specification, the term “Group III nitride compound semiconductor” also encompasses Group III nitride compound semiconductors which are doped with an impurity for determining a conduction type of p or n.
2. Background Art
Group III nitride compound semiconductors are direct transition semiconductor, and a light-emitting element formed from a Group III nitride compound semiconductor emits light having a wavelength ranging from ultraviolet to red. Therefore, Group III nitride compound semiconductors have been employed for producing light-emitting elements such as a light-emitting diode (LED) and a laser diode (LD). Since a Group III nitride compound semiconductor has a large band gap, an element produced from the semiconductor is considered to be operated reliably at high temperature, as compared with an element produced from a semiconductor other than a Group III nitride compound semiconductor. Therefore, applications of Group III nitride compound semiconductors to a variety of transistors, including an FET, have been developed. Since arsenic (As) is not contained in Group III nitride compound semiconductors as a major component thereof, from the environmental viewpoint, use of the semiconductors in a variety of semiconductor elements is envisaged. In general, a Group III nitride compound semiconductor is formed on a sapphire substrate.
However, when a Group III nitride compound semiconductor is formed on a sapphire substrate, misfit dislocations are generated due to the difference in lattice constant between sapphire and the Group III nitride compound semiconductor, thereby deteriorating properties of the resultant semiconductor element. Such misfit dislocations thread through semiconductor layers in a direction perpendicular to the substrate, and the dislocations (105 to 1010 dislocations per cm2) propagate throughout the Group III nitride compound semiconductor. The dislocations propagate through Group III nitride compound semiconductor layers of different compositions to the uppermost layer. When light-emitting elements such as an LD or an LED are produced from the Group III nitride compound semiconductor, because of propagation of the dislocations, properties of the element (e.g., threshold current of LD, service life of LD, or service life of LED) are impaired. Meanwhile, when semiconductor elements other than light-emitting elements are produced from the Group III nitride compound semiconductor, scattering of electrons caused by the dislocations (i.e., crystal defects) imparts low mobility to the resultant semiconductor element. Such problems arise even when the Group III nitride compound semiconductor is formed on another type of substrate.
Generation of dislocations will be described with reference to a schematic representation shown in FIG. 3. FIG. 3 shows a substrate 91, a buffer layer 92 formed on the substrate 91, and a Group III nitride compound semiconductor layer 93 formed on the buffer layer 92. Conventionally, the substrate 91 has been formed from, for example, sapphire; and the buffer layer 92 has been formed from, for example, aluminum nitride (AlN). The aluminum nitride (AlN) buffer layer 92 is provided for mitigating misfit between the sapphire substrate 91 and the Group III nitride compound semiconductor layer 93. However, even when the buffer layer 92 is provided, generation of dislocations cannot be completely prevented. Threading dislocations 901 propagate in a vertical direction (i.e., a direction perpendicular to the substrate) from dislocation start points 900, and the dislocations 901 thread through the buffer layer 92 and the Group III nitride compound semiconductor layer 93. When a desired Group III nitride compound semiconductor is laminated on the upper surface of the Group III nitride compound semiconductor layer 93, to thereby form a semiconductor element, threading dislocations propagate in a vertical direction from dislocation start points 902 on the upper surface of the Group III nitride compound semiconductor layer 93 through the resultant semiconductor element. Thus, through conventional techniques, propagation of dislocations cannot be prevented during formation of a Group III nitride compound semiconductor layer.